Ballparks are just as trendy as clothes and hairstyles. In the 1960s and '70s, many cities built doughnut-shaped multipurpose stadiums. It was also the era of the domed stadium. We were in awe of the immensity of the Astrodome in Houston and the Kingdome in Seattle. Today, a new trend in stadiums has led to the demolition of the Kingdome and the likely demise of the Astrodome. The latest trend in sports stadiums is the retractable-roof stadium.

These new convertible stadiums allow athletic fields to be covered during inclement weather, but can be opened to allow the sun to shine in on fair weather days. This new type of stadium has all but killed off the idea of artificial turf stadiums, which cause a greater number of injuries than natural turf. Retractable roofs are an idea whose time has come, but just how do they work? There's no single answer to that question because no two retractable roof stadiums are exactly alike. Let's look at a few of these convertible stadiums:

Skydome, Toronto -- The home of the Blue Jays of Major League Baseball (MLB) was opened in 1989 and began the era of retractable roof stadiums. While other stadiums had previously been built with removable tops, a la Montreal's Olympic Stadium, the Skydome was the first to have a fully retractable roof. The roof consists of four steel panels and it moves along rails at a rate of 71 feet (21 meters) per minute and takes 20 minutes to open or close. When closing the roof, 72 motors are used to move the two panels backward and forward over a fixed panel like a telescope and a fourth panel rotates 180 degrees to completely close the roof.

Bank One Ballpark, Phoenix -- Home to the Diamondbacks of Major League Baseball, Bank One Ballpark, or "the BOB," opened in 1998. In the searing heat of the Arizona desert, it was almost a necessity to have a ballpark that could be open during moderate days and closed and air-conditioned on hot summer days. Bank One's roof uses a system similar to a drawbridge. A pair of 200 horsepower motors drive the roof panels to open and close in just four minutes using four miles of cable strung through a pulley system.

Safeco Field, Seattle -- In 1999, baseball's Mariners said good-bye to the Kingdome and moved to Safeco. Safeco's roof is divided into three independent panels made of a thin gypsum and glass fiber board. A waterproof skin covers the panels. The three panels are staggered so that the roof can slide completely away from the field of play. The panels roll on motorized steel wheels that move over steel rails. The wheels are powered by 96 10-horsepower, DC motors. Power is provided by long cables that are reeled in and out as the roof slides open and shut. Opening and closing the roof is done at a rate of 30 feet (9.1 m) per minute and takes about 10 to 20 minutes.

Minute Maid Park, Houston -- Opened in 2000 as Enron Field (after the Enron scandal it was temporarily known as Astros Field before Minute Maid bought naming rights), Minute Maid Park is the home of baseball's Astros. The newest of the retractable-roof stadiums, it consists of three panels with staggered heights that slide completely off of the top of the playing field. Even the glass walls on one side of the stadium retract to allow baseballs to be hit competently out of the stadium. Sixty electric motors with 7.5 horsepower drive 140 36-inch (91 cm) steel wheels that slide over tracks on the east and west sides of the stadium. Opening and closing takes between 12 and 20 minutes.

Reliant Stadium, Houston -- Reliant opened in 2003 as the home of the Texans, a National Football League (NFL) expansion team. Reliant's 956 x 385-foot (291 x 117 m) roof divides into two panels that open in the middle of the stadium over the 50-yard line. The panels are made out of a translucent Teflon-coated fiberglass. Five trichord trusses support each panel and span between supertrusses. The panels will slide over rails built on top of the supertrusses at a rate of about 35 feet (10.7 m) per minute.

Retractable roofs are really marvels of modern architecture. They are designed to move panels that weigh millions of pounds in just minutes. By doing so, they give fans a new experience at baseball and football games. Given their popularity, it's likely we will see many more of these convertible stadiums built in the next decade until they are replaced by the next stadium trend.

Earl Campbell won the 1977 Heisman Trophy and was inducted into the Hall of Fame in 1990. His career was cut short due to the constant pounding and abuse he took during his eight-year career. Learn about Earl Campbell's great running back career.